Add Flutter SDK Management Support (Similar to FVM)

[jjdevzinho]

Hello! I'd like to suggest adding support for Flutter SDK management in FlyEnv, similar to FVM (Flutter Version Management). The idea is to allow FlyEnv to:

• Download and manage multiple Flutter SDK versions
• Set specific versions per project
• Automatically configure PATH for selected versions
• Easily update or switch SDK versions

This would be a great addition for mobile developers working with Flutter and would make FlyEnv even more versatile as a full-stack environment manager.

Thanks for the amazing work on FlyEnv! 👏